1. Field of the Invention
This invention relates to a system for the concentration of adsorbates by thermally reversed adsorption of adsorbates of various kinds, solutes and gases, from solvents or other fluid carriers. A relatively warm fluid containing the adsorbate is fed to an initially relatively cooler adsorbent bed. As the warmer fluid moves through the bed a high pulse in concentration of the adsorbate develops immediately ahead of the thermal front, if the bed properties are appropriate. To insure the desired properties of the bed, the thermal (or adsorptive) properties of the bed (in respect to the adsorbate of interest) are modified such that at a mean temperature the solute adsorbed per unit volume in the solid, n, divided by that in the fluid, c, (for c.fwdarw.0) is approximately equal to the volume heat capacity of the solid .rho..sub.s c.sub.p.sbsb.s divided by that of the fluid .rho.c.sub.p. [Note: when heats of adsorption contribute materially they will modify the ##EQU1## ratio]. The ratio of these ratios ##EQU2## is a function of concentration c and temperature. The packed bed is chosen such that ##EQU3## at the initial bed temperature (as c.fwdarw.0) and becomes ##EQU4## at the elevated concentration desired. The fluid is introduced at a temperature such that ##EQU5## at the initial concentration.
A sharp thermal wave is generated as the hot fluid enters the cool bed. The concentration in the pulse of adsorbate immediately ahead of the wave, c.sub.2 (at T2) is related to solute concentration in the inlet stream, c.sub.i, by: ##EQU6## Note that ##EQU7## is evaluated at the lower temperature and the concentration c.sub.2. The width of the pulse is proportional to the mass that has passed though the thermal front. There is a sharp drop in concentration at the forward edge of this concentration pulse. This pulse in solute concentration is harvested by aspirating fluid from the bed at the time the pulse arrives at the end of the bed.
Following the discharge of the high concentration pulse from the bed, the bed is found to be filled with fluid at the inlet concentration and temperature in approximate equilibrium with the solid. When the flow of fluid, free of the removed adsorbate, is passed back through the bed at the cool temperature, a concentration wave will precede the thermal wave toward the original bed inlet, if the appropriate value of n/c and ##EQU8## have been selected. All the solution that leaves the bed before the concentration wave reaches the inlet is recycled. When the thermal wave reaches the original inlet, the bed has been regeneratively recharged in respect to adsorbate as well as temperature with the exception of the imbalance caused by the harvested fluid concentrate. Provisions for this mass flow and hence heat flow imbalance are proposed. The general requirements for the thermal and physical properties and operating conditions that permit this recycling process to be accomplished are described.
2. The Prior Art
Activated charcoal is used in many clean-up processes and has been applied at low temperature in the removal of urea and other unwanted solutes from the dialysate of an artificial kidney (1) (FIG. 5 of Giordano et al, page 40, "Annual Report on Oxystarch in Uremia" to the National Institute of Arthritis and Metabolic Diseases, Artificial Kidney-Chronic Uremia Program, Jan. 19, 1975, to July 18, 1976. ) The cyclical use of two temperatures to achieve purification or concentration has been published by Wilhelm and his colleagues (2) (Ind. Eng. Chem. Fundamentals, Vol. 7, p. 337, 1968 and U.S. Pat. No. 3,369,874.)
The present invention is more nearly related to some aspects of high temperature chromatography. (3) (Rhee et al., "An Analysis of an Adiabatic Adsorption Column Part IV: Adsorption in the High Temperature Range." The Chemical Engineering Journal, Vol. 3, pp. 121-135, 1972.) In example 5 of reference (3) the initially cool packed bed was fed wlith hot N.sub.2 carrying benzene. It was shown that under certain conditions:
a. A pulse of benzene could be generated having a higher concentration than the inlet concentration and would move with the thermal wave velocity. PA1 b. The concentration behind the pulse returned to the inlet concentration. PA1 1. The use of charcoal column at low temperatures to adsorb urea, creatinine, and presumably other unwanted solutes at 0.degree. C. from precooled dialysate (Giordano et al.) (1). Here the charcoal can be regenerated by purging with warm water and reused. The important contribution of Giordano et al is the demonstration that the adsorption isotherms for urea and creatinine on a representative charcoal are significantly different at 37.degree. C. and 0.degree. C. PA1 2. A technology that could be employed but has not yet been employed for this purpose is parametric pumping (Wilhelm et al.) (2) wherein solute concentrations can be raised on one side of a column, reduced on the other side by the synchronous alternations of flow direction and bed temperature. The similarity rests in reliance on the change in adsorptivity with temperature. PA1 3. Rhee et al. (3) described the fundamental phenomena that are responsible for the effect exploited in the current design. The system described by Rhee et al is a gas-solid system where adsorption reversal is easily attained. The possibility of thermally modifying the bed material to insure adsorption reversal at the desired temperature range has not been discussed so far as is known. PA1 4.
The example was constructed for an adiabatic system in which a solute adsorbed on the solid according to Langmuir adsorption. The example conditions were selected such that adsorption reversal occurred: the bed was initially cool such than n/c exceeded (.rho.c.sub.p).sub.s /(.rho.c.sub.p) and was fed with a hot fluid such that n/c was less than (.rho.c.sub.p).sub.s /(.rho.c.sub.p). The present invention pertains to those beds that exploit the concentrating potential of the adsorption reversal condition either through the selection of the temperature range or the thermal modification of the bed to bring ##EQU9## close to unity.
In the experiments of Popovich et al. (4) ("Physiological Transport Parameters in Patients in Peritoneal and Hemodialysis," Contract NO1-AM-3-2205. Proceedings of the Ninth Annual Contractors' Conference of the Artificial Kidney Program of the National Institute of Arthritis, Metabolism, and Digestive Diseases, pp. 126-128b, 1976.) it was shown that if 20 liters per day of dialysate were equilibrated with the blood through peritoneal lavage throughout a virtually 24 hour period (with a net water removal rate of approximately two liters per day), the uremic patient could be maintained indefinitely. It was felt in these preliminary considerations that an adsorbent that could concentrate and remove the unwanted solutes from the fluid draining from the peritoneal cavity could reduce the amount of fluid required, and thus the overall mass could be carried by the patient. Preliminary calculations based on thermal energy storage experiments and, in particular, a recent analytical paper by Riaz (5) ("Transient Analysis of Packed-Bed Thermal Storage Systems." presented at the International Solar Energy Society annual meeting, Orlando, Fla., June 7, 1977.) lead to the realization that the propagation velocity of the thermal wave could be modified such that adsorption reversal could occur for a wide range of adsorbates and adsorbents employing the data of Giordano et al. (1) and (6) (Sparks et al, "Adsorption of Nitrogenous Waste Metabolites from Artificial Kidney Dialyzing Fluid." Chemical Engineering in Medicine, Chemical Engineering Progress Symposium Series, No. 66, Vol. 62, pp. 2-10.) Preliminary calculations showed that a system to concentrate urea by adsorption reversal could be accomplished by adding non-adsorbing thermal sinks to the bed.
Giordano et al. (1) and Sparks et al. (6) have shown that adsorption of urea on charcoal is reversible (as are most physical adsorptions) and the bed can be washed for reuse by flushing with a clean solvent. Experience with storage heat exchangers has shown that they can be restored to the initial temperature through purging.
The need to restore the bed to its initial conditions of concentration and temperature with a minimum of added fluid in the artificial kidney problem led to the design of a doubly regenerative system that concentrates a solute, then cools and cleans the bed for reuse without the need for any extra purging fluid. This system, with respect to urea, acts as an artificial loop of Henle.
In the treatment of uremic patients ultrafiltrate purification and reintroduction is not practiced at present. Current research that appears in the literature includes:
Popovich (4) has employed peritoneal lavage virtually around the clock to establish the potential efficacy of continuous solute removal from dialysate in maintaining a uremic patient.